Help me identify this algae.

[Thealchemist7]

I've had this tank for about 2 years now. It's always been stable as far as algae and I've never really done any major testing or dosing. Until a couple months ago (maybe 3 months) it was fine. But now it's began to develop this brown algae over the rocks. While my coral and fish seem happy and healthy it's bugging me. To make matters worse it's not the type you can scrub off, I've tried and it isn't even phased by five minutes of brushing. So what kinda algae is it? Sorry I can't provide any water chemistry number but I'm sure you guys can narrow down what it is.

Salt
red sea coral pro salt.
RODI water. TDS reading always zero.